1. Field of the Invention
The present invention relates to a differential gear apparatus provided with a differential limitation mechanism, and more specifically to a structure of the differential gear apparatus of this type.
2. Description of the Prior Art
When an automotive vehicle changes its travelling direction or travels along a curve, an appropriate differential motion is required for the right and left vehicle wheels via a differential gear apparatus for providing smooth vehicle travelling. However, when the vehicle is running on a road whose surface friction coefficient is small such as a muddy road and therefore one of the right and left vehicle wheels slips, since no drive power is transmitted to the other of the two vehicle wheels because of its construction, the vehicle cannot get out of the muddy road. To overcome the above-mentioned problem, a differential gear apparatus provided with a differential limitation mechanism has been proposed, in which when one of the vehicle wheels slips on a muddy road and therefore a differential operation is produced between the two vehicle wheels, the differential operation is limited to drive the other non-slipping wheel for facilitation of vehicle's getting out of a muddy road.
FIG. 1 shows one of these prior-art differential gear apparatus provided with a differential limitation mechanism, by way of example, which is disclosed by the same applicant in Japanese Published Unexamined (Kokai) Utility Model appli. No. 62-85748. In FIG. 1, a ring gear (not shown) is fitted to a differential case 101 of the differential gear apparatus. With this ring gear, a drive pinion gear (not shown) is engaged to transmit a rotative power from an internal combustion engine to this ring gear. The differential case 101 is formed with an engage groove 102 engaged with a pinion shaft 103. A pinion gear 104 is rotatably supported by the pinion shaft 103, and in mesh with a pair of side gears 105a and 105b. The side gear 105a is spline-coupled to a right side wheel drive shaft 106 arranged coaxially with a left side wheel drive shaft 107. Between the right side wheel drive shaft 106 and the left side wheel drive shaft 107, a viscous coupling 108 housed within a differential case 101 is disposed. The viscous coupling 108 comprises a coupling case 109, a hub member 110, etc., and a working chamber 111 is partitioned by the coupling case 109 and the hub member 110. The working chamber 111 is filled with a viscous fluid and provided with first resistance plates 112 circumferentially splined coupled to the coupling case 109 and second resistance plates 113 also circumferentially spline coupled to the hub member 110. Further, the coupling case 109 is spline coupled to the left side wheel drive shaft 107 and the hub member 110 is spline coupled to the right side wheel drive shaft 106. Furthermore, a washer 114 is interposed between the coupling case 109 and the differential case 101.
When a differential operation is produced between the left side wheel drive shaft 107 and the right side wheel drive shaft 106 on a muddy road, the viscous fluid is sheared between the first resistance plates 112 and the second resistance plates 113. Therefore, the differential operation is limited by a shearing resistance generated at the first resistance plates 112 and the second resistance plates 113, so that the vehicle is allowed to get out of a muddy road.
In the prior-art differential gear apparatus provided with a differential limitation mechanism as described above, however, when a differential operation is produced between the left side wheel drive wheel 107 and the right side wheel drive wheel 106 and therefore the viscous fluid is sheared by the first resistance plates 112 and the second resistance plates 113, the viscous fluid is heated and therefore expanded. Once the viscous fluid is expanded, since the coupling case 109 is elastically deformed into expansion, the washer 114 interposed between the differential case 101 and the coupling case 109 is urged against the differential case 101. Therefore, there exists a problem in that the washer 114 is seized by heat and therefore the differential case 101 and the coupling case 109 are not rotatable relative to each other.